Faster-Than-Light Neutrinos? Physics Luminaries Voice Doubts

A few dozen nanoseconds, an imperceptibly slim interval in everyday life, can make all the difference in experimental physics. A European physics collaboration made a stunning announcement September 23, after having clocked elementary particles called neutrinos making the underground journey from a lab in Switzerland to one in Italy. The neutrinos made the trip 60 nanoseconds faster than they would have traveling at light speed, the researchers found. Faster, that is, than the rules of physics as we understand them would allow.

Astrophysicist and cosmologist Martin Rees of the University of Cambridge
Extraordinary claims require extraordinary evidence. I think it will be perceived in retrospect as an embarrassment that this claim received so much publicity—the inevitable consequence of posting a preprint on the Web. Neutrinos were observed from SN 1987A more or less coincidentally with the explosion—not four years earlier, as would have been the case if the velocity difference had been the same as is now claimed (though, of course, the energies of the supernova neutrinos are much lower).

Theoretical physicist Steven Weinberg of the University of Texas at Austin, winner of the 1979 Nobel Prize in Physics
The report of this experiment is pretty impressive, but it bothers me that there is plenty of evidence that all sorts of other particles never travel faster than light, while observations of neutrinos are exceptionally difficult.* It is as if someone said that there are fairies in the bottom of their garden, but they can only be seen on dark, foggy nights.

Theoretical physicist Lawrence Krauss of Arizona State University
It is an embarrassment as far as I am concerned. It was not unreasonable for the experimentalists to submit a paper with an unexplained result. But a press conference on a result, which is extremely unlikely to be correct, before the paper has been refereed, is very unfortunate—for CERN and for science. Once it is shown to be wrong, everyone loses credibility. Neutrino experiments are hard, and systematic errors at the limit of resolution can be significant. Moreover, because the experiment appears to violate Lorentz invariance, which is at the heart of so much known physics, one should be skeptical. One should be additionally skeptical because observations of SN 1987A showed, as I wrote in 1998, that neutrinos and photons travel at the same speed to one part in a billion, several orders of magnitude below the claimed effect. Now, the only way out of that is to have some energy-dependent effect, but all the ones that make sense don't wash here.

Neutrino physicist Karsten Heeger of the University of Wisconsin–Madison
I think it is fair to say that many of us are stunned. This is an extraordinary result and we cannot even perceive all the implications if this result were true. All of modern physics as we know and teach is based on Einstein's special and general theories of relativity. If this effect were shown to be true, I would consider this the physics revolution of the century.

As an experimentalist, we are all very cautious of such extraordinary results. I watched the Webcast from CERN ... and was impressed by the detailed and careful work presented by the OPERA collaboration in their talk. However, even for a neutrino expert it is difficult to follow and understand all the technical details that enter this measurement. There could be systematic effects that have not been accounted for. Independent experimental tests are needed.

Theoretical physicist Alexander Kusenko of the University of California, Los Angeles
While this experimental group is very good and competent, neutrino experiments are notoriously difficult to perform. This experiment is particularly ambitious. One has to wait for a confirmation.

Neutrinos are fast, but science news travel faster!

Theoretical physicist Heinrich Päs of the Technical University Dortmund in Germany
It seems that the experimentalists were very careful, but this is really BIG news....

There are certain misunderstandings with people who are very cynical now: Even if true, this result neither proves Einstein wrong nor implies that causality has to be violated and time travel is possible. Things can move faster than the speed of light without violating Einstein if either the speed of light is not the limiting velocity as one can observe it for light propagation in media such as, for example, water. This can be modeled with background fields in the vacuum as has been proposed by [Indiana University physicist] Alan Kostelecky.

Or spacetime could be warped in a way so that neutrinos can take a shortcut without really being faster than the speed of light. As our three space plus one time dimensions look pretty flat, this would require an extra dimension (as proposed by [University of Hawaii at Manoa physicist] Sandip Pakvasa, [Vanderbilt University physicist] Tom Weiler and myself).

On the other hand, if something moves faster than the speed of light, causality violations—aka time travel—may be a possibility (for example, in models with two warped extra dimensions as proposed by [Vanderbilt physicist] James Dent, Pakvasa, Weiler and myself). And that, of course, would have really crazy and mind-boggling consequences, but even there can [there] be scenarios which are contradictory.

So, in short, this is really exciting. But since it is so exciting, I'm not sure whether one should be too optimistic that it will survive the tests of other experiments.

* Editor's note (9/27/11): The first sentence of this response was updated after publication to reflect new information from Weinberg.